While metformin is often prescribed to women with PCOS in some countries, it is not routinely prescribed for the condition within the UK.

In this study, Norwegian researchers examined 487 pregnant women with PCOS, a condition which is linked with increased the risk of infertility and adverse pregnancy outcomes including gestational diabetes. The average age of the women was 29 years.

The participants were randomised to receive either daily metformin or placebo from their first trimester through to birth, without knowing which drug they received.

A combined incidence of miscarriage during the second trimester and premature birth at less than 37 weeks was almost halved among the metformin group. Those who adhered to the treatment throughout pregnancy had a particularly beneficial effect. A total of 81% took more than 80% of their medication.

Researchers also observed similar rates of gestational diabetes between the two groups, suggesting metformin had no effect in lowering risk.

"This finding is disappointing but not surprising," said lead author Tone Loevvik, M.D., a Ph.D. student at Norwegian University of Science and Technology. "It is in accordance with the results from two previous studies from our research group."

Metformin treatment was associated with less weight gain though, which Dr Loevvik added is positive for prospectively helping to lower BMI during pregnancy and reduce the risk of complications.